The present invention relates to a hydraulic system of an automatic transmission, and more particularly to a hydraulic system for controlling a change in hydraulic fluid pressure upon not only supply of hydraulic fluid to, but also discharge thereof from a hydraulically actuable frictional coupling of an automatic transmission.
Japanese Patent First (unexamined) Publication No. 1-116352 discloses a hydraulic system which comprises a hydraulically actuable frictional coupling, a shift valve arranged to receive a pressurized hydraulic fluid from a source of pressurized hydraulic fluid, i.e., a line pressure regulator, a first fluid passage having one end connected to the shift valve and an opposite end, and a pressure regulator valve having an inlet port connected to the opposite end of the first fluid passage. An outlet port of the pressure regulator is connected to one end of a second fluid passage having an opposite end connected to the coupling. The pressure regulator valve has a drain port and a signal pressure port connected to a pressure modifier valve which generates a throttle valve opening related pressure which is called a pressure modifier pressure. A pressure accumulator is operatively coupled with the pressure regulator and has a reciprocal piston defining a variable volume chamber communicating with the second fluid passage at a portion between one end and an opposite end thereof. A common spring operatively disposed between the reciprocal piston and a spool of the pressure regulator valve to increase a spring force with which the spring biases the spool during stroke of the reciprocal piston upon supply of hydraulic fluid to the coupling. In order to discharge hydraulic fluid from the second fluid passage directly to the first fluid passage bypassing the pressure regulator valve, a one-way valve is fluidly disposed between the first fluid passage and the second fluid passage. Thus, a pressure variation characteristic during a shift to a release or a disengaged position of the coupling is not controlled by the pressure regulator valve.
An object of the present invention is to improve a hydraulic system of the above kind such that the pressure regulator valve can control a pressure variation characteristic during a shift to the release or the disengaged position of the hydraulically actuable frictional coupling.